Hippomenes was a legacy of Poseidon and the husband of Atalanta.
Percy Jackson's Greek Heroes
Hippomenes wandered into Arcadia one day and wondered what a crowd of people were gathering for. When he learned they were exited about see the princess kill a suitor in a foot race, he became appalled until he saw the princess and started flirting with her and, to everyone’s surprise, was not killed. She invited him back the following week to race her.
After the encounter went to the temple of Aphrodite and cleaned it up and prayed to the love goddess. She gave him three apples from her sacred tree and told Hippomenes to drop them at certain points during the race and to give her the proper sacrifice afterwards. He did as followed and he and Atalanta were married that same day and possibly fathered her son Partenopeus.
However, he never made the sacrifice and a year after their wedding they were returned from an outing when Aphrodite filled them with lust as they were passing a temple of Zeus and kissed passionately. The King of the Olympians saw this and turned the two into lions.
Appearance
In Percy Jackson's Greek Heroes, Hippomenes was described to be an extremely handsome young man: curly black hair, sea-green eyes, strong graceful limbs, and a beautiful voice that caught Atalanta's particular attention.